1. Field of the Invention
The present invention relates to a printing type drum and, more particularly, to a printing type drum for use in such a small printer which prints by striking its printing type belts with hammers from inside of the printing type drum.
2. Description of the Prior Art
Typical construction of a conventional printing type drum that contains hammers therein is known to be as illustrated in FIG. 1, FIG. 2 and FIG. 3 which is disclosed in U.S. patent application Ser. No. 155,157, filed May 30, 1980 now abandoned and refiled as continuation 407,296 on Aug. 11, 1982 and which is owned by the assignee of the present invention.
In the figures, the printing type drum, whose whole is designated by the reference numeral 1, is basically comprised of a cylindrically formed drum body 2. Grooves 2a are formed in the periphery of the drum body 2, spaced side by side in the axial direction of the drum body 2. These circular grooves 2a are demarcated by ridges 2b. Each of the grooves 2a has through holes 2c formed therein circumferentially in a queue at given intervals.
Printing type belts 3 are fitted in the grooves 2a of the drum body 2. Each printing type belt 3 is formed of an elastic material, such as NBR or urethane. On the surface thereof are provided raised type blocks 3a in a queue at a given spacing. Projection 3b are provided on the back of the printing type belt 3 in counter positions to the printing type blocks 3a. When the type belt 3 is fitted in the groove 2a of the said drum body 2, the projections 3b on the back of the type belt 3 are also fitted in the through holes 2c of the groove 2a, whereby the type belt 3 is secured to the drum body 2. In many cases, the type belt 3 is in a circular form with the ends thereof joined by a proper method.
A hammer unit 4 is accommodated in the drum body 2 installed with the type belts 3. The hammer unit 4 is basically constituted of a long and slender framework assembled with a pair of upper and lower support frames 5 joined at their ends 5a. Inside the support frames 5 are securely installed a plurality of hammer assemblies 6, spaced at given intervals, in the longitudinal direction of the support frames. As is clearly noted from the sectional view in FIG. 3, each of the hammer assemblies 6 has a yoke 7. The yoke 7 has a -shaped section and contains a coil 8 therein. A rod 9 is fitted in the center hole of the circularly formed coil 8 in such a manner that it is free to slide. The back end of the rod 9 is connected to one end of a guide rod 11 with the medium of a bracket 10. The guide rod 11 is fitted to the yoke 7 in parallel with the said rod 9 so as to slide freely, with a spring 12 provided between the outside of the yoke 7 and the bracket 10. And a hammer 13 is fixed to the forward end of the guide rod 11. The hammer 13 is constantly under the recoiling force of the spring 12 with the tendency to move away from the projections 3b on the back of the type belt 3.
As is clearly noted from FIG. 2, two hammers 13 are provided and protrude as a pair on the base 14 secured to the end of the guide rod 11, one each at the left and the right thereof. This provision is to allow two hammers to be operated with one hammer assembly. Corresponding with this, as shown in FIG. 1, the type belt 3 is attached one pitch behind or ahead of adjacent type belts so that the type blocks 3a adjacent to each other, and consequently the projections 3b adjacent to each other, are staggered. Of course, the through holes 2c formed in the grooves 2a of the drum body 2 are also staggered.
Now, in front of the printing type drum 1 having such a construction as has been described above, there is located a platen 15, and the printing paper 16 is allowed to run down through between the platen and paper guides 17 and 18 as it is printed, and is discharged by a drive roller 19 and a pinch roller 20.
An ink roller 21 is provided opposite to the platen 15, with the printing type drum 1 sandwiched, in such a position that it is always pressed against the type belt 3 under a given pressure.
In a printer equipped with the printing type drum having such a construction as has been described above, the printing type drum is rotated by means of a control unit not illustrated here. When the type block 3a of a selected type belt comes to a position facing the platen 15, the coil 8 is energized to produce a magnetic field. As a result, the rod 9 is moved toward the platen 15, overcoming the recoiling force of the spring 12, to cause the hammer 13 to strike the platen 15 via the typing paper 16, whereby the printing is accomplished. When the printing is finished, the hammer 13 is retracted by the recoiling force of the spring 12 and the type block 3a is returned to the original position by the elasticity that the type belt 3 itself possesses.
However, if such a construction of the printing type drum as has been described above is adopted, where an ink-moistened printing ink roller 21 is constantly pressed against the type belt, there arises a problem in that the ink is caused to invade the gap between the type belt 3 and the groove 2a of the drum body 5. This problem occurs by the capillarity developed because of the gap between the type belt 3 and the groove 2a.
The printing ink contained in the ink roller 21, being a liquid, is solidified when dried and produces an adhesion at the same time, resulting in the type belt 3 sticking to the groove 2a of the drum body 2.
In a printing type drum having built-in printing hammers, if such a phenomenon as has been described above occurs, a small amount of energy is no longer sufficient enough to continue printing while the type belt sticking to the groove is being removed, and a tremendous amount of energy is required to drive the hammers properly. Of necessity, therefore, a large hammer unit is required, and the printing type drum itself must be large in size, too. An energy too small for the hammering operation would result in a print with missing letters or incompletely typed letters.